Rheumatoid arthritis (RA) is a chronic inflammatory arthritis marked by synovial hyperplasia with local invasion of bone and cartilage. Accumulating evidence suggests that RA fibroblast- like synoviocytes (FLS), which form the leading destructive front of rheumatoid synovium, possess unique characteristics and contribute to cartilage degradation. Previous studies demonstrated that cytokines activate Jun N-terminal kinase (JNK) in FLS, that this kinase is phosphorylated in RA synovium, and that JNK regulates joint damage in animal models of arthritis. Two upstream kinases, MKK4 and MKK7, can independently activate JNK and initiate protease and cytokine gene expression. In contrast to this shared function in other cell types, our preliminary data suggests that MKK7 is the primary pathway for cytokine- and Toll-like receptor (TLR) 2 and 4--mediated JNK activation in synoviocytes and that MKK4 only participates in limited responses to certain cellular stresses (e.g., TLR3 activation and anisomysin). Over-activation of JNK in RA synovium and synoviocytes might be due, in part, to deficient expression Gadd45ss, which can function as an endogenous MKK7-JNK inhibitor. We hypothesize that MKK7 is a potential therapeutic target that modulates pro-inflammatory gene expression in synoviocytes, thereby influencing a subset of JNK functions involved in the pathogenesis of RA, while leaving other functions regulated by MKK4 in intact. To test this hypothesis, we will determine the role of MKK4, MKK7, and Gadd45ss in murine models of inflammatory arthritis. Second, we will explore the mechanisms and functional sequelae of MKK7 activation in cultured synoviocytes. Third, we will examine how Gadd45ss and upstream kinases like TAK 1 regulate MKK4 and MKK7 activation in FLS. These data will test the hypothesis that the JNK pathway plays a pivotal role in the synoviocyte biology and is a potential target for chondroprotective therapy. PUBLIC HEALTH RELEVANCE. Fibroblast-like synoviocytes (FLS) in rheumatoid arthritis play a major role in joint destruction by virtue of their ability to produce proteases and cytokines. This process is mediated, in part, by signal transduction pathways like c-Jun N-terminal kinase (JNK). This project will explore how JNK in RA is regulated by upstream kinases and determine the potential for inhibiting one of these signaling molecules as a therapeutic target.